Extraction with 2M TFA (Trifluoroacetic acid). TFA has been commonly used to
extract As species in rice plant samples (Abedin et al., 2002; Liu et al., 2006;
Williams et al., 2005). The sample (0.2000 g) was weighed into the quartz glass
digestion tubes, and steeped in 2 ml of 2 mol/l TFA solution. The tubes were
then left overnight at room temperature, followed by digestion heating block at
100C for 5 h until the solution evaporated to dryness. If not, the temperature
was elevated to 160C to dry the content. When the tubes were cooled down,
the residues and solution were vortexed by ultra-pure water to 10 ml. Samples
were then centrifuged at 7000 rpm for 8 min, and the supernatant was
collected and passed through a 0.45 mm 13 mm nylon filter.